Oxidation of low-density lipoprotein (LDL) occurs due to excessive active oxygen, namely oxidative stress conditions, caused by disruption of the balance between generation and elimination of active oxygen in vivo. LDL is a giant molecule having a conjugate between apolipoprotein B and lipids such as cholesterol, phospholipid, and triglyceride. First, active oxygen oxidizes unsaturated fatty acid in the component lipids during the LDL oxidation. Next, a subsequent chain oxidation reaction occurs, followed by undergoing conjugated diene synthesis to generate lipid peroxide and aldehyde. This chain oxidation reaction or a direct oxidation reaction due to active oxygen also oxidizes apolipoprotein B to be cleaved. These oxidation reactions result in loss of the spherical structure of LDL, an increase in negative charge, and an altered affinity for receptors. This can produce oxidized LDL having characteristics different from those of native-LDL.
Oxidized LDL is present in atherosclerotic lesions, and is detected at a high concentration in sera from patients with hyperlipidemia, diabetes mellitus, or liver disease, etc., when compared with those from healthy subjects. Thus, oxidized LDL is considered to be an important substance in various diseases involved in oxidative stress. In order to be used for elucidation, treatment, diagnosis, and evaluation of these diseases, oxidized LDL-specific antibodies have been developed.
Examples of the antibodies can include: an anti-oxidized-phospholipid antibody that is produced by using, as an immunogen, oxidized LDL purified from atherosclerotic lesions and that recognizes, as an antigen, phospholipid having oxidatively cleaved fatty acid (Non Patent Literature 1); an anti-MDA-LDL antibody that is produced by using, as an immunogen, malondialdehyde-modified LDL (MDA-LDL) prepared from native-LDL in serum (Non Patent Literatures 2 and 3); and an antibody that is produced by preparing acetylated LDL, MDA-LDL, and metal-oxidized LDL from native-LDL in serum and by using, as an immunogen, a solution containing an equivalent amount of each of them (Patent Literature 1).
LDL is a giant particle having many sites susceptible to oxidation, so that the degree of the oxidation varies. In general, a large number of antioxidants such as vitamin C are present in blood in vivo. Oxidized LDL with a high degree of oxidation (highly oxidized LDL) is recognized as a foreign substance, and is thus rapidly removed by macrophages. Hence, oxidized LDL with a low degree of oxidation (slightly oxidized LDL) accounts for a large proportion of oxidized LDL present in blood, in particular. Meanwhile, highly oxidized LDL seems to account for a large proportion of oxidized LDL present in vascular walls and atherosclerotic lesions. The above-described reason seems to explain why the antibody disclosed in Non Patent Literature 1 is an antibody against highly oxidized LDL, but not against slightly oxidized LDL.
In addition, since in the antibodies disclosed in Non Patent Literatures 2 and 3, an antigen recognition site is present inside of LDL, a sample must undergo pretreatment to expose its antigenic site.
Furthermore, any of the antibodies disclosed in Non Patent Literatures 1 to 3 and Patent Literature 1 has not been produced using, as an immunogen, oxidized LDL present in serum. Consequently, when they are used for a serum or plasma sample, a false positive reaction occurs and their sensitivity is low. It is therefore difficult to detect effects resulting from drug administration or improvement in lifestyle.